RADIATIVE ASSOCIATION REACTIONS OF SILICON AND TRANSITION-METAL CATIONS WITH THE PAH COMPOUNDS BENZENE, NAPHTHALENE, AND ANTHRACENE

Using the ion cyclotron resonance ion trap, reactions of Si+ with benz ene, naphthalene, and anthracene were studied under low-pressure condi tions of interest in interstellar modeling. Radiative association was efficient with all three hydrocarbons, occurring on roughly one-quarte r of collisions. Association competed with charge transfer and/or cond ensation reactions in all three cases. In the naphthalene case it was inferred from collision-induced dissociation evidence and from reactio n behavior that the association product ion at low pressure rearranges to a sigma complex or other intimately bonded structure, in contrast to the pi complex assigned under the high-pressure conditions in the S IFT. Reactions of the transition-metal ions Fe+, Cr+, and Mn+ with nap hthalene were studied for comparison. Radiative association was also e fficient for these ions. Association of the transition metal-naphthale ne adducts with a further naphthalene molecule to yield the ''sandwich '' complex was also observed, although this type of complex was not fo rmed for Si+ reacting with naphthalene.